This invention relates to a cage-roll unit for use in a welded metal pipe forming mill.
Known methods of continuously forming metal sheet, such as steel strip, into a cylindrical form as a step for manufacturing steel pipe include step roll forming which carries out stepwise forming of the strip combining horizontal top and bottom rolls and vertical side rolls and cage-roll forming which comprises pre-forming into an arc having an approximately uniform curvature throughout its entirety by horizontal, curved top and bottom rolls, then forming the edges by edge-forming rolls.
Pipe forming mills based on these known methods, especially those of the side-roll type, each have vertical shafts on both sides of bottom rolls that form a plane along which the piece is threaded, with side rolls rotatably carried by the shafts to deform the piece from both sides.
The side- and other rolls used for shaping the flat piece into a round form have grooves, which define shapes among each set of rolls which are known as passes, designed in accordance with the diameter of pipe to be manufactured. When the pipe diameter is changed from one to another, the rolls are changed to those which form passes suited for the latter, with the roll spaces accordingly adjusted.
Each side-roll unit of this type has a pair of vertical shafts which carry replaceable rolls of different diameters, from rather large ones to small ones. In terms of the outside diameter of the pipe, the largest one is 3 to 4 times larger than the smallest one. (In the case of medium-size pipe, the diameter ranges from 8 inches minimum to 24 inches maximum.) As will be understood, intervals at which the side-roll units are positioned along the mill axis must be large enough to accommodate the side rolls for the largest-diameter pipe. Therefore, when the side rolls are changed to those for the smallest-diameter pipe, with the intervals of the side-roll units unchanged, excess space is left between the side rolls. Unless such excess space which exists during the small-diameter pipe forming is filled, poor threading or excessive stepped forming might result, entailing twisting or deformation of product pipe.
It is conceivable to replace some of the side rolls with many cage rolls, in an attempt to manufacture good-shaped pipes over a wide diameter range by the step roll forming method. But a cage-roll unit occupies a larger space than a side-roll unit in the direction of the mill axis. For example, a cage-roll forming mill is approximately 1.5 times longer than a step roll forming mill. Accordingly, it is difficult to replace part or all of the side-roll units in an existing step roll forming mill with cage-roll units of the conventional type.
When the diameter of pipe to be manufactured changes, the space between the side rolls on both sides of the mill axis must be ajusted accordingly. This space adjustment is relatively easy with the side-roll unit as compared with the cage-roll unit. Therefore, if the side-roll and cage-roll units are used selectively for the desired pipe diameters, production efficiency can be increased without marring the form of product pipe. To permit this, the two units must be readily interchangeable.
U.S. Pat. No. 3,472,053 exemplifies a cage-roll mill of the conventional design, with many cage-roll units, each carrying a pair of cage rolls, disposed along the mill axis.